1. Field of the Invention
This invention relates to a thermal recording device of direct heat-sensitive type or thermal transfer type, and particularly to a thermal recording device by which a possibility of appearance of a white space line which is produced on a recording medium in the case when waiting time for information to be inputted to the recording device is comparatively long can be eliminated.
2. Description of the Prior Art
FIG. 1 is a schematic side view showing a conventional thermal recording device which has heretofore been utilized in general wherein a thermal recording paper 1 is held between a drive roll 2 and a pinch roll 3, and the recording paper 1 is supplied from a supply roll 4 by the rotation of these rolls in the direction of the arrow, so that the recording paper travels between a recording head 5 and a back roll 6 opposed thereto. As is well known, the recording head 5 is provided with a number of heat generating elements, and every 1 line thermal recording is executed by such a manner that electrical energy corresponding to 1 line recording information to be recorded is supplied to individual heat generating elements.
The thermal recording paper 1 is further wound up by means of a winding roll 7.
In such thermal recording device as mentioned above, however, the thermal recording paper 1 is conveyed in a condition where it is stretched by means of the drive roll 2 and the pinch roll 3 in ordinary recording state.
For this reason, the thermal recording paper 1 used to be conveyed under such conditions where the recording paper has been stretched by a certain extent. Moreover, in this case, there is a certain amount of strain on rubber portion around the outer boundary of the drive roll 2 in the circumferential direction thereof. In normal operating conditions where printing is successively executed for the respective lines with an interval within a predetermined period, there is no substantial obstacle in respect of the above extent of the recording paper stretched as well as the amount of strain around the circumference of the drive roll, because they have constant values, respectively.
However, when waiting time for inputting printing information is prolonged so that stop time in rotation of the drive roll 2 is also prolonged, the rubber portion of the drive roll 2 tends to return to its equilibrium state so that the circumferential rubber portion is displaced towards the direction for drawing out the recording paper 1, whilst the recording paper 1 in the stretched state tends to return to its normal state.
In this case, the thermal recording paper 1 is firmly held and clamped in abutting portion between the drive roll 2 and the pinch roll 3 in general. Thus, the thermal recording paper 1 shrinks towards the direction of the abutting portion between the drive roll 2 and the pinch roll 3 so that a portion of the thermal recording paper 1 existing in between the thermal head 5 and the back roll 6 moves along its travelling direction.
As a result, as is clearly understood from FIG. 2, a pitch a between lines in travelling direction of record corresponding to the aforesaid portion of the recording paper becomes larger than a normal pitch b to produce a white space line c, so that it brings about a disadvantage of deterioration in picture quality. In FIG. 2, mark o designates a recording dot produced by the thermal head 5, and a dot column #1 is recorded in accordance with first line buffer information, whilst a dot column #2 is recorded in accordance with memory information of second line buffer.